Vinegar residue supported nanoscale zero-valent iron: Remediation of hexavalent chromium in soil

Environmental Pollution xxx (xxxx) xxx

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Vinegar residue supported nanoscale zero-valent iron: Remediation of hexavalent chromium in soil* Guangpeng Pei a, b, Yuen Zhu a, Junguo Wen a, Yanxi Pei c, Hua Li a, * a

School of Environment Science and Resources, Shanxi University, Taiyuan, Shanxi 030006, China Institute of Resources and Environment Engineering, Shanxi University, Taiyuan, Shanxi 030006, China c School of Life Science, Shanxi University, Taiyuan, Shanxi 030006, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2019 Received in revised form 26 September 2019 Accepted 14 October 2019 Available online xxx

A composite material comprising of nanoscale zero-valent iron (nZVI) supported on vinegar residue (nZVI@VR) was prepared and applied for remediation of soils contaminated by hexavalent chromium (Cr(VI)). Sedimentation test results revealed that the nZVI@VR displayed enhanced stability in comparison to the bare-nZVI. Remediation experiments exhibited the immobilization efficiency of Cr(VI) and Crtotal was 98.68% and 92.09%, respectively, when using 10 g nZVI@VR (nZVI 5%) per 200 g Crcontaminated soil (198.20 mg kg
1 Cr(VI), 387.24 mg kg
1 Crtotal) after two weeks of incubation. Further analyses demonstrated that almost all the exchangeable Cr was transformed into FeeMn oxide bound and organic matter bound. Moreover, the application of nZVI@VR enhanced soil organic carbon content and reduced redox potential. After granulation, the immobilization efficiency of Cr(VI) and Crtotal achieved 100% and 91.83% at a dosage of 10% granular nZVI@VR. Granular nZVI@VR also accelerated the transform of more available Cr (exchangeable and bound to carbonates) into less available fractions (Fe eMn oxide bound and organic matter bound), thus resulting in a remarkable reduction in the Cr bioavailability. These results prove that nZVI@VR can be an effective remediation reagent for soils contaminated by Cr(VI). © 2019 Elsevier Ltd. All rights reserved.

Keywords: Vinegar residue Nanoscale zero-valent iron Cr (VI)-Contaminated soil Granulation Remediation

1. Introduction Hexavalent chromium (Cr(VI)) is one of the most toxic and carcinogenic compounds known to humans (Kazakis et al., 2018; Su et al., 2016a). A large amount of Cr(VI) produced through various industrial processes like wood preservation, leather tanning, electroplating and agricultural activities of sludge and sewage irrigation is discharged into soil (Reyhanitabar et al., 2012; Su et al., 2016b; Su and Fang, 2015), which poses a threat to human health. The United States Environmental Protection Agency (EPA) categorizes Cr a priority pollutant (US EPA, 1992a). In 2014, China released the national bulletin of soil pollution survey that indicated that 16.1% of its land was polluted, out of which 1.1% of sampling points were polluted with Cr that exceeded the standard rate (Ministry of environmental protection and ministry of land and resources of the

* This paper has been recommended for acceptance by Dr. Yong Sik Ok. * Corresponding author. School of Environmental Science and Resources, Shanxi University, Taiyuan 030006, China. E-mail address: [email protected] (H. Li).

People’s Republic of China, 2014). Therefore, it is urgent to develop an effective measure to remediate the Cr(VI) contaminated soil. More recently, nanoscale zero-valent iron (nZVI) has been advanced as an ideal remediation material for in situ remediation of contaminated soil and groundwater, especially for heavy metals pollutants (Lefevre et al., 2016). Because of its large surface area, fast reactive rate and reduction capability, nZVI is used as a reactive media for remediation of Cr(VI) (Fang et al., 2011; Mahmoud et al., 2013). Nonetheless, there are still some technical challenges that need to be resolved for its wide application (Jiang et al., 2018). In practical applications, the effect of nZVI on the remediation of pollutants is reduced because of the influence of problems such as aggregation (Wei et al., 2019), poor transport capabilities (Lefevre et al., 2016), passivation (Zou et al., 2019), and reduced electron transfer (Oh et al., 2017). To solve these problems, various studies have been conducted based on the modification of nZVI such as sulfidation (Cao et al., 2017), surfactants (Tian et al., 2018), and noble metal doping (Wu et al., 2018). However, these methods may inhibit the reaction of nZVI with target pollutants or lead to new environmental problems (Wang et al., 2019). For example, sulfidation may not be widely applied because it causes corrosion of the

https://doi.org/10.1016/j.envpol.2019.113407 0269-7491/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Pei, G et al., Vinegar residue supported nanoscale zero-valent iron: Remediation of hexavalent chromium in soil, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113407

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previous studies (Shang et al., 2017; Yan et al., 2015). Briefly, 1.25 g of ferrous sulfate (FeSO4$7H2O) and 4.75 g of VR were combined in 100 mL of mixed solution (ethanol/deionized water, 4/1) and agitated with magnetic stirring at 200 rpm at room temperature for 24 h. Then, 12.5 mL of KBH4 solution (20 g L
1) was added (dropwise, 1e2 drops min
1) into the slurry under magnetic stirring (200 rpm). nZVI@VR samples were then separated from the mixture by vacuum filtration and rinsed with ethanol several times. nZVI@VR was then vacuum dried at 60  C. The loading amount of nZVI was 5%. Nitrogen (N2) was continuously injected in the preparation process to avoid oxidation of nZVI@VR. In addition, bare-nZVI and nZVI@VR with different loading amount of nZVI (10, 15, 20%) were also prepared in the same way as above. The specific surface areas (SSAs) of the VR, bare-nZVI, and nZVI@VR were measured by an ASAP 2020 analyzer (Micromeritics, U.S.A.). The morphologies of VR and nZVI@VR were imaged by scanning electron microscopy (SEM, SU-70, Hitachi, Japan). The minerals in VR, nZVI, and nZVI@VR were analyzed by X-ray diffraction (XRD, D8 VENTURE, Bruker, U.S.A.) and data were collected the 2q range from 10 to 80 . The functional groups on the surface of the materials were characterized by fourier transform infrared spectroscopy (FTIR, Nicolet iS 50, Thermo Scientific, U.S.A.).

nZVI core and increases the particle size (Rajajayavel and Ghoshal, 2015). Applying bimetallic particles may also enhance corrosion, which can reduce the lifetime of nZVI, and may introduce additional heavy metal pollution. Another approach that would achieve this goal is the immobilization of nZVI in a support matrix, including biochar (Wang et al., 2019), silica (Wu et al., 2019), kaolin (Wang et al., 2016), montmorillonite (Arancibia-Miranda et al., 2016), and zeolites (Huang et al., 2019). In recent years, biochar has attracted widespread attention as a supporting matrix for nZVI (Wang et al., 2019; Su et al., 2016a). Combined with nZVI, biochar can provide additional benefits, such as the sorbing contaminants, reduction of redox-sensitive contaminants, and enhancement of nZVI reactivity (Oh et al., 2017). However, the synthesis of biochar can be costly due to the additional energy consumption (e.g., nitrogen or electricity), which will increase the cost of pollutant remediation. Therefore, a good support material should be cheap, safe, and able to reduce the negative effects caused by bare-nZVI treatment. Vinegar residue (VR), as a by-product of vinegar production, is mainly composed of vinegar fermentation by-products and additives (rice chaff, bran and sorghum shell) (Lu et al., 2013). About 3.2 million tons of VR is produced every year in China (Feng et al., 2017). A study by Song et al. (2012) has shown that VR contains abundant organic acids, such as acetic acid, lactic acid, malic acid, and tartaric acid. The crude fiber content of VR from corn and sorghum reached 344 and 348 g kg
1, respectively (Wang et al., 2010). In the last few years, VR has been gradually applied to the remediation process of environmental pollution. Pei et al. (2017) produced a new soil amendment with VR, weathered coal and stainless steel slag, which effectively facilitated adsorption and immobilization of lead (Pb) in soil. Another study demonstrated that biochar produced from VR had progressed the immobilization effect on Cadmium (Cd) in soil (Li et al., 2018). However, to date very few studies are available regarding the use of VR as supporting material for nZVI. Therefore, the present study was conducted to: (1) prepare and characterize VR supported nanoscale zero-valent iron (nZVI@VR); (2) evaluate and compare the stability between nZVI@VR and barenZVI; (3) investigate the immobilization of Cr, and the changes of Cr species in soil; and (4) study the effect of granulation of nZVI@VR on the Cr immobilization efficiency.

2.3. Sedimentation analysis The stability of nZVI@VR and bare-nZVI were determined by sedimentation analysis (Liang et al., 2014; Liu et al., 2016). Briefly, nZVI@VR or bare-nZVI suspensions (1 g L
1) were prepared and transferred into 100 mL glass tubes. The suspension was sonicated for 5 min and then the optical density was measured at 508 nm using an UVeVis spectrophotometer (UVe5100B, Metash instrument Co., Ltd, China). 2.4. Immobilization of Cr(VI) in soil The uncontaminated surface soil was sampled from the farmland in Yuci, Shanxi Province, China. Soil samples were air dried at room temperature and passed through to a 2 mm sieve. Contaminated soil was prepared by dissolving K2Cr2O7 in water and mixing the liquid with the soil. The samples were then placed in a jar and incubated at room temperature for 90 d to obtain heavily Cr(VI)contaminated soil. The final contents of Cr(VI) and Crtotal in the soil were 198.20 and 387.24 mg kg
1. The soil had a pH of 7.85, a cation exchange capacity (CEC) of 22.09 cmol kg
1, a redox potential (Eh) of 138 mV and total organic carbon (TOC) of 7.87 g kg
1. To compare the performance of nZVI@VR, bare-nZVI and VR for immobilization of Cr(VI), experiments were carried out in a plastic pot (diameter 8.5 cm, height 9.0 cm). There were four treatments. In treatment 1, 200 g of Cr-contaminated soil was treated with 9.50 g of VR, 0.50 g of nZVI, and 10 g of nZVI@VR (nZVI 5%). In treatment 2, 200 g of Cr-contaminated soil was treated with 9 g of VR, 1 g of nZVI, and 10 g of nZVI@VR (nZVI 10%). In treatment 3, 200 g of Cr-contaminated soil was treated with 8.50 g of VR, 1.50 g of nZVI, and 10 g of nZVI@VR (nZVI 15%). In treatment 4, 200 g of Crcontaminated soil was treated with 8 g of VR, 2 g of nZVI, and 10 g of nZVI@VR (nZVI 20%). A Cr-contaminated soil (200 g) without any residue was used as a control (CK). The moisture content of the

2. Materials and methods 2.1. VR samples VR was collected from a vinegar factory in Shouyang, Shanxi Province, China. VR was finely ground and sieved with a standard 0.149 mm sieve. The basic properties of VR in this study are shown in Table 1. The attapulgite was obtained from the Ministry of Ecology and Environment at the Nanjing Institute of Environmental Sciences of China. In this study all the chemicals were analytical grade or of equivalent quality. 2.2. Preparation and characterization of nZVI@VR nZVI@VR was prepared following the method developed by

Table 1 Basic properties of vinegar residue in this study. Heavy metal contents (mg kg
1)

Materials

pH

Elemental analysis (%) C

H

O

N

S

Pb

Cr

Cd

As

Hg

Vinegar residue

5.47

43.20

5.57

47.79

3.03

0.42

49.84

0.02

0.04

0.04

1.20

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mixtures was adjusted to 60% water holding capacity and all the samples were incubated at 25  C for two weeks. There were three replicates in each experiment. To compare the leachability of Cr in the untreated and treated soil, toxicity characteristic leaching procedure (TCLP) test was used to investigate the immobilization efficiency of nZVI@VR on Cr (US EPA, 1992b). In the TCLP tests, as the ratio of solid-liquid is 1:20, 1 g of soil was extracted in a 20 mL extraction fluid (fluid No.1, pH 4.93 ± 0.05) on a rotating shaker for 18 h at 25  C. Then, the immobilization efficiency was calculated as follows:

 Cr immobilization efficiency ¼

1


m m0

  100%

(1)

Where m is the Cr(VI) or Crtotal quantity (mg) in the supernatant of the treated soil, and m0 is the Cr(VI) or Crtotal quantity (mg) in the supernatant of the blank group tests. To estimate the change of Cr species in soil after treated by nZVI, VR and nZVI@VR, sequential extraction procedure (SEP) was performed as per the method presented by Tessier et al. (1979). According to its solubility in different solvents, Cr was extracted sequentially and was partitioned into five fractions, including an exchangeable fraction (EX); a fraction bound to carbonates (CB); a fraction bound to FeeMn oxide bound (OX); an organic matter bound (OM) fraction; and a residual (RS) bound fraction. The relative availability and toxicity of the five fractions was EX > CB > OX > OM > RS (Wang et al., 2014a). The contents of Crtotal and Cr(VI) in soil were measured by digesting the soil in acid and alkaline solutions (Wang et al., 2014b). The Crtotal content in TCLP leachate was determined by the atomic absorption spectrophotometer (AAS, 140/240, Varian Spectrum, USA). The Cr(VI) content in TCLP leachate was determined by the diphenylcarbazide colorimetric method using an ultravioletevisible spectrophotometer (UVe5100B, Shanghai Metash Instruments Co., Ltd., China) following the method developed by Environmental Protection Standard (China GB 7467-87). 2.5. Soil characteristics after remediation To estimate the changes of physical and chemical properties of the soil samples before and after remediation, pH, CEC, TOC, Eh and available iron were tested and evaluated. The soil pH value was measured using a pH meter at a soil:water ratio of 1:2.5. The amounts of TOC values were determined by the potassium dichromate method (Bao, 2016). The CEC values were determined by the sodium acetate method (Bao, 2016). Soil Eh were measured using the potential method (China HJ 746e2015). To study iron accumulation in soil treated by nZVI@VR, the content of available iron in soil was determined by diethylene triamine penlaacetic acid (DTPA) (Bao, 2016). Briefly, 25 g of soil sample was homogenized with 50 mL DTPA solution (pH 7.30) in a conical glass flask. The mixtures were extracted on a rotating shaker (180 rmin) at 25  C for 2 h. After that, the mixtures were filtered with 0.22 mm membranes. Then iron content in the filtrate was measured using AAS at 248.3 nm. 2.6. Granulation experiments Considering the slow release properties of particle remediation materials, granulation experiments was conducted to explore the effect of granular nZVI@VR on the immobilization of Cr(VI) in soil. According to our previous study, attapulgite was chosen as the cementitious material for granulation. In brief, nZVI@VR was mixed with attapulgite at ratio of 17:3 (w:w) and then water, equaling 80% of the total volume, was added and stirred until the mixture was

3

completely homogenized. Granulation of nZVI@VR was completed by an extrusion granulator (JZL-80, Changzhou Yongchang Granulating Drying Equipment Co., Ltd., China) under room temperature, and then dried using a blast drier at 50  C. An incubation experiment was conducted to study the effect of granular nZVI@VR on Cr immobilization efficiency. Plastic pots (diameter 8.5 cm, height 9.0 cm) were filled with 200 g of soil, and treated with 1, 3, 5, and 10% of granular nZVI@VR. In addition, a blank control without granular nZVI@VR was used for comparison. All the treatments were performed at a constant humidity (60% maximum water holding capacity) and incubated at 25  C for two weeks. All the treatments were performed in triplicate. After incubation, the soil was collected and air-dried at room temperature to analyze Cr immobilization efficiency and the speciation of Cr in soil.

2.7. Statistical analysis One-way variance analysis (ANOVA) with least-significant difference was used to determine the statistical significance. The statistical significance in this analysis was defined at p < 0.05.

3. Results and discussion 3.1. Characterization of nZVI@VR The SSA of bare-nZVI, VR and nZVI@VR (nZVI 20%) were all determined by a BET apparatus. Results indicated that the SSA value of bare-nZVI, VR and nZVI@VR (nZVI 20%) were 4.70, 5.23 and 8.37 m2 g
1. The surface area of nZVI@VR was observed to be higher than that of bare-nZVI and VR. To observe the surface morphology, SEM images of VR, and nZVI@VR were obtained (Fig. 1A and B). The SEM images showed that the surface of VR was smooth, with some small cracks (Fig. 1A). As shown in Fig. 1B, nZVI particles loaded on the VR appeared to be spherical and less reunited, with sizes of about 400 nm. The smooth surface of the VR may be more suitable for the distribution of nZVI, which increased their probability of participating in the reaction. For biochar, with its large surface area and abundant pore structure, most of the nZVI particles were distributed on the inner surface of the pores, with a low probability of participating in the reaction (Dong et al., 2017a; Zhu et al., 2009). In addition, an XRD analysis of the samples was conducted (Fig. 1C). It can be seen that the XRD spectra of nZVI and nZVI@VR contained an apparent characteristic peak of Fe0. For nZVI, the peaks at 35.26 and 60.04 (2q) corresponded to Fe2O3 and Fe3O4, respectively. The results showed the presence of small quantities of iron oxides on the surface of nZVI and nZVI@VR. An FTIR spectra analysis of the sample was conducted to determine the distinct surface functional groups in the range of 400e4000 cm
1 (Fig. 1D). Typically, the peak at about 3440 cm
1 was related to the stretching vibration in the eOH (Su et al., 2016b). This indicated the presence of some hydroxyl groups on the surface of the nZVI, VR and nZVI@VR. The peak at approximately 2940 cm
1 was related to the eCH3 group and the peak at wavenumber near 2860 cm
1 was related to the eCH2 group, which were attributed to long-chain aliphatic components (Das et al., 2009). The peak at about 1636 cm
1 was related to the aromatic C]C (Qian et al., 2017). The bands around 1380 and 1100 cm
1 were related to the COO- group and CeO bending vibration, respectively (Dong et al., 2017b). The peak at about 610 cm
1 of VR and nZVI was attributed to FeeOeH, indicating that chemical bonds existed between the nZVI and VR (Yan et al., 2015).

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Fig. 1. Characterization of the synthesized materials: SEM image (A, VR; B, nZVI@VR); (C) XRD image; (D) FTIR spectra. nZVI, nanoscale zero-valent iron. VR, vinegar residue. nZVI@VR, vinegar residue supported nanoscale zero-valent iron.

3.2. Sedimentation of nZVI@VR The stability of bare-nZVI and nZVI@VR with different loading amounts of nZVI (5, 10, 15, 20%) were qualitatively characterized by sedimentation tests (Fig. 2). It was observed that the absorbance of nZVI and nZVI@VR decreased as the time increased. For bare-nZVI, nZVI@VR (nZVI 5%), nZVI@VR (nZVI 10%), nZVI@VR (nZVI 15%) and nZVI@VR (nZVI 20%), the absorbance at 508 nm was reduced by 80.60, 72.87, 75.40, 76.87, and 78.40%, respectively, in 60 min. In particular, at 4 min, the absorbance of the nZVI and four nZVI@VRs had decreased by 65.36, 20.97, 20.00, 26.13 and 29.30%,

respectively. The settlement rate of nZVI was the fastest. This was because nZVI particles can form micrometer-scale or lager aggregates in less time, leading to more sedimentation (Ruiz-Torres et al., 2018). The settlement rate of nZVI@VR was slower than that of nZVI, indicating that the loading of nZVI onto VR restrained them from aggregating and improved their dispersity. In Fig. 2, it was observed that nZVI@VR (nZVI 5%) had the highest stability, followed by nZVI@VR (nZVI 10%), nZVI@VR (nZVI 15%) and nZVI@VR (nZVI 20%). This means that the settlement was related to the loading amount of nZVI, in which the less the loading amount, the longer it takes for aggregation, followed by less sedimentation. This result was in accordance with the stability of biochar-supported nZVI (Su et al., 2016a). A plausible reason for this phenomenon may be because the nZVI was immobilized on the VR’s micropore surface by the oxygen-containing functional groups, or the electrostatic attraction between nZVI particles was reduced by the VR’s surface charge characteristics. 3.3. Remediation effect of nZVI@VR on Cr-contaminated soil

Fig. 2. Sedimentation curves of bara-nZVI and nZVI@VR. nZVI, nanoscale zero-valent iron. nZVI@VR, vinegar residue supported nanoscale zero-valent iron.

The bare-nZVI, VR and four kinds of nZVI@VR was used to remediate Cr-contaminated soil. As illustrated in Fig. 3, the immobilization efficiency of nZVI@VR (nZVI 5%), nZVI@VR (nZVI 10%), nZVI@VR (nZVI 15%) and nZVI@VR (nZVI 20%) for Cr(VI) was 98.68, 97.30, 97.20, and 96.89%, respectively, after two weeks of incubation. The immobilization efficiency of the four kinds of nZVI@VR for Crtotal was 92.09, 92.89, 92.51, and 92.71%, respectively. As the loading amount of nZVI increased from 5 to 20%, the immobilization efficiency of Cr(VI) decreased slowly from 98.68 to 96.89%, but the immobilization efficiency of Crtotal remained 92%. This could be ascribed to the increased nZVI loading in the nZVI@VR composites. The nZVI particles were more easily aggregated, and thus provided less reactive sites for Cr(VI) (Dong et al., 2017a). A

Please cite this article as: Pei, G et al., Vinegar residue supported nanoscale zero-valent iron: Remediation of hexavalent chromium in soil, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113407

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Fig. 3. Immobilization efficiency of Cr in soil samples with different treatments: (A) Treatment 1; (B) Treatment 2; (C) Treatment 3; (D) Treatment 4. The different lowercase letters indicate significant difference between different treatments (p < 0.05). nZVI, nanoscale zero-valent iron. VR, vinegar residue. nZVI@VR, vinegar residue supported nanoscale zerovalent iron.

larger VR loading resulted in a larger surface area, leading to most of the nZVI particles being distributed on the surface of VR, with a high probability of participating in the reaction. Therefore, the results indicated that a low loading amount of nZVI (5%) could achieve a good Cr(VI) and Crtotal immobilization efficiency. The results indicated that the immobilization efficiencies of Cr(VI) and Crtotal were similar with the different nZVI@VR treatments. However, these results were obviously different in the nZVI and VR treatments. As Fig. 3A shows, the VR treatment had significantly higher Cr(VI) and Crtotal immobilization efficiency than that the nZVI treatment (p < 0.05). Nevertheless, the immobilization efficiency increased with increasing nZVI dosage, and decreased with decreasing VR dosage (Fig. 3) (p < 0.05). These observations indicated that nZVI or VR alone can promote the immobilization efficiency of Cr in the soil. However, when nZVI and VR were combined to synthesize nZVI@VR, the immobilization efficiency of Cr was significantly higher than that of nZVI or VR alone (Fig. 3). A similar phenomenon was observed in the remediation process of Cr-contaminated soil by biochar-supported nZVI (Su et al., 2016b). The interaction mechanism of nZVI-based materials with heavy metal ions is still uncertain. However, the main interaction

mechanisms could be regarded to be adsorption, reduction, and precipitation (Zou et al., 2016). Based on the experimental results and previous studies, Cr(VI) immobilization by nZVI@VR could occur as follows. (1) Adsorption of Cr(VI) onto the nZVI@VR surface via surface pores and oxygen-containing functional groups (CeO, COO-, and eOH) (Lyu et al., 2018). (2) Reduction of Cr(VI) by Fe0 (Eqs. (2)e(4)) (Li et al., 2015). (3) Precipitation of (CrxFe1-x)(OH)3 and CrxFe1-xOOH (Eqs. (5) and (6)) (Li et al., 2015; Zhao et al., 2017).

3þ 0 þ 2þ 2HCrO
4 ðaqÞ þ 3Fe ðsÞ þ 14H ðaqÞ/3Fe ðaqÞ þ 2Cr ðaqÞ

þ 8H2 OðlÞ (2)

3þ 2þ þ 3þ HCrO
4 ðaqÞ þ 3Fe ðaqÞ þ 3H ðaqÞ/3Fe ðaqÞ þ Cr ðaqÞ

þ 2H2 OðlÞ (3)

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3þ þ 3þ 3Fe2þ ðaqÞ þ CrO2
4 ðaqÞ þ 8H ðaqÞ/3Fe ðaqÞ þ Cr ðaqÞ

þ 4H2 OðlÞ (4)

2017). The EX in the VR treated soil decreased to 11.57%, while OX and OM increased to 41.12 and 23.83% (Fig. 4A). This change may be attributed to the oxygen-containing functional groups of VR, which facilitate the sorption of Cr(VI) onto the VR surface (Lyu et al., 2016; Su et al., 2016a). After nZVI@VR remediation, the CB fraction was reduced to almost zero. However, compared to nZVI@VR, nZVI showed no significant reduction in the CB fraction or an increase in the CB fraction with VR (p > 0.05). These results indicated that the

ð1
xÞFe3þ ðaqÞ þ xCr3þ ðaqÞ þ 3H2 OðaqÞ / ðCrx Fe1
x ÞðOHÞ3 ð0 < x < 1ÞðsÞ þ 3Hþ ðaqÞ

(5)

ð1
xÞFe3þ ðaqÞ þ xCr3þ ðaqÞ þ 2H2 OðaqÞ / Crx Fe1
x OOHð0 < x < 1ÞðsÞ þ 3Hþ ðaqÞ

(6)

VR can provide additional organic matter, which can affect nZVI functions. Organic matter can enhance the solubility of organic compounds, form strong complexes with metals, and then change the adsorption/immobilization of the metals (Zhao et al., 2016). The functional group sites provided by VR were the main driving forces for complexation (Eqs. (7)e(9)), such as, eOH, eCOOH and eC]O can be used as electron donors for Cr (VI) reduction and can participate in Cr complexation (Hou et al., 2019; Ma et al., 2019).

reduction of the CB fraction in the nZVI@VR treatment was due to the combined action of nZVI and VR. The results showed that nZVI@VR could promote the conversion of more easily available Cr (EX and CB) into less available (OX and OM), thus, leading to an apparent decrease in the bioavailability of Cr. These results were attributed to a synergistic effect on the adsorption coupled reduction over the nZVI@VR composite (Zhang et al., 2019).

3þ þ 3R
OHðsÞ þ HCrO
4 ðaqÞ þ 4H ðaqÞ/3R
OðsÞ þ Cr ðaqÞ

3.5. Influence of nZVI@VR on soil properties

þ 4H2 OðlÞ (7) 3R
COOHðsÞ þ Cr3þ ðaqÞ / ðR"TT5843c571""ADCOOÞ3 CrðsÞ þ 3Hþ ðaqÞ (8) 3
R
C ¼ OðsÞ þ HCrO
4 ðaqÞ/R
C ¼ O…HOCrO ðsÞ

(9)

Previous research has suggested that acidic conditions are more favorable for the reduction of Cr by nZVI (Wang et al., 2019; Zhu et al., 2017). In alkaline soils, the high OH
content can corrode the surface of nano-iron particles, and there was not enough Hþ to eliminate this corrosion (Hou et al., 2019). In this study, the soil pH was 7.85, which was not conducive to the reduction ability of nZVI. However, VR, as the support matrix, provided a localized acidic environment for nZVI, which would enhance the reactivity of nZVI. However, nZVI could improve the reactivity, surface texture, and magnetism of VR (Ho et al., 2017). Therefore, nZVI had a synergistic action with VR in the reduction and precipitation process of Cr. 3.4. Speciation of Cr in soil The SEP analysis was conducted to explore the relative availability of Cr in soils before and after nZVI, VR and nZVI@VR treatment (Fig. 4). For untreated soil, the Cr species were EX (32.98%), CB (3.77%), OX (30.56%), OM (10.17%), and RS (22.50%). In comparison to the untreated soil, the EX fraction in four kinds of nZVI@VR treatments were almost completely converted to OX, with OX increased by 84.24, 99.25, 110.39, and 103.61% (p < 0.05). The results may be attributed to the formation of Cr2O3, Cr(OH)3, and Cr(III)/Fe(III) oxides/hydroxides (Lyu et al., 2018; Wang et al., 2014a). Additionally, the OX fraction in nZVI treatments was significantly higher than in the CK (p < 0.05), which might be due to the formation of (CrxFe1
x)(OH)3 and CrxFe1
xOOH (Zhang et al.,

Selected physical-chemical properties of soils are presented in Table 2. The pH of soil is an important factor for immobilization effect of various heavy metals in soils (Jiang et al., 2018). Compared to the pH values of CK (7.73), the pH of the nZVI-amended soil increased slightly (7.79e7.85). This slight increase in pH could be due to nZVI corrosion and Cr(VI) reduction (Gheju, 2011). However, acidity of VR could also result in decrease of the soil pH. Therefore, the pH of the nZVI@VR-amended soil decreased with low loading amounts of nZVI, but increased with high loading amounts of nZVI. Compared to CK, nZVI addition had a negative effect on the soil CEC, but VR had a positive effect on the soil CEC. No statistically significant effect was observed after nZVI@VR addition (p > 0.05). An addition of VR and nZVI@VR significantly increased the soil TOC contents (p < 0.05) (Table 2). For example, the TOC content increased from 8.10 g kg
1 for CK to 21.13 g kg
1 for nZVI@VR (5%) amended soil. However, the content of TOC in nZVI@VR-amended soil decreased as the loading amount of nZVI increased. This is because the content of VR decreased relatively in nZVI@VR. The results thus indicated that nZVI@VR could effectively improve the soil content of organic carbon and attribute to VR that is rich in biomass (Wang et al., 2015). Previous studies showed that soil redox potential has an important effect on the valence and morphological transformation of Cr in soil (Ho et al., 2017; Li et al., 2015). In this experiment, Table 2 shows addition of nZVI to Cr(VI)-contaminated soil significantly decreases Eh of soil. The addition of nZVI@VR could also significantly reduce the soil Eh values. The Eh values of nZVI@VRamended soil were lower than those of nZVI-amended soil. This could be because VR provided protection for the strong reducibility of nZVI in soil. Iron is one of the most important elements for soil organisms, plant growth and mammalian cells. Table 2 shows the available iron for CK was 3.21 ± 0.06 mg kg
1. The available iron content was higher in treated soil than in the CK as because the content increased as the nZVI dosage increased. The reason for this phenomenon was basically attributed to excessive iron released after

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Fig. 4. Changes in Cr speciation in soil samples with different treatments: (A) Treatment 1; (B) Treatment 2; (C) Treatment 3; (D) Treatment 4. nZVI, nanoscale zero-valent iron. VR, vinegar residue. nZVI@VR, vinegar residue supported nanoscale zero-valent iron. EX, exchangeable; CB, bound to carbonates; OX, FeeMn oxide bound; OM, organic matter bound; RS, residual.

Table 2 Selected physio-chemical properties in soils.

pH

CEC (cmol kg
1)

TOC (g kg
1)

Eh (mV)

Available iron (mg kg
1)

Treatment

CK

Treatment 1

Treatment 2

Treatment 3

Treatment 4

nZVI VR nZVI@VR nZVI VR nZVI@VR nZVI VR nZVI@VR nZVI VR nZVI@VR nZVI VR nZVI@VR

7.73 ± 0.01Ab 7.73 ± 0.01Aa 7.73 ± 0.01Ac 21.63 ± 0.28Aa 21.63 ± 0.28Ab 21.63 ± 0.28Aa 8.10 ± 0.40Aa 8.10 ± 0.40Ab 8.10 ± 0.40Ad 135.00 ± 0.58Aa 135.00 ± 0.58Aab 135.00 ± 0.58Aa 3.29 ± 0.23Ae 3.29 ± 0.23Aa 3.29 ± 0.23Ad

7.79 ± 0.02Aa 7.63 ± 0.00Bc 7.58 ± 0.01Cd 20.03 ± 0.32Bb 22.65 ± 0.06Aa 22.35 ± 0.40Aa 7.75 ± 0.27Ca 23.50 ± 0.17Aa 21.13 ± 0.40Ba 127.00 ± 1.53Bc 136.33 ± 1.20Aa 121.00 ± 1.15Cb 13.86 ± 0.89Ad 3.35 ± 0.42Ca 4.14 ± 0.52Bc

7.80 ± 0.02Aa 7.64 ± 0.01Bbc 7.73 ± 0.04Abc 19.05 ± 0.22Bc 21.81 ± 0.12Ab 21.76 ± 0.51Aa 7.57 ± 0.58Ca 23.06 ± 0.18Aa 20.90 ± 0.61Ba 127.67 ± 0.58Bc 132.00 ± 0.58Abc 120.67 ± 1.20Cb 23.71 ± 0.63Ac 2.33 ± 0.58Ba 8.19 ± 0.46Bc

7.79 ± 0.03Aa 7.67 ± 0.01Bb 7.79 ± 0.01Ab 18.77 ± 0.18Bc 22.69 ± 0.12Aa 21.72 ± 1.72ABa 7.41 ± 0.14Ca 22.98 ± 0.22Aa 16.10 ± 0.40Bb 122.00 ± 1.15Ad 121.33 ± 1.86Ad 109.00 ± 0.58Bc 37.07 ± 1.63Ab 3.30 ± 0.13Ca 14.31 ± 0.13Bb

7.85 ± 0.01Aa 7.71 ± 0.01Ba 7.86 ± 0.01Aa 18.70 ± 0.42Bc 22.69 ± 0.24Aa 22.62 ± 0.00Aa 7.27 ± 0.18Ca 22.95 ± 0.23Aa 14.48 ± 0.61Bc 131.33 ± 1.33Ab 129.67 ± 1.20Ac 109.67 ± 0.33Bc 44.74 ± 0.59Aa 3.30 ± 0.21Ba 19.15 ± 0.52Ba

Data are presented as the means ± standard error (SE) of three replications. CK, a Cr-contaminated soil (200 g) without any treatment as a control. Treatment 1, 200 g of Crcontaminated soil was treated with 9.50 g of VR, 0.50 g of nZVI, and 10 g of nZVI@VR (nZVI 5%). Treatment 2, 200 g of Cr-contaminated soil was treated with 9 g of VR, 1 g of nZVI, and 10 g of nZVI@VR (nZVI 10%). Treatment 3, 200 g of Cr-contaminated soil was treated with 8.50 g of VR, 1.50 g of nZVI, and 10 g of nZVI@VR (nZVI 15%). Treatment 4, 200 g of Cr-contaminated soil was treated with 8 g of VR, 2 g of nZVI, and 10 g of nZVI@VR (nZVI 20%). Different capital letters in the same column indicate a significant difference (p < 0.05). Different lowercase letters in the same row indicate a significant difference (p < 0.05). nZVI, nanoscale zero-valent iron. VR, vinegar residue. nZVI@VR, vinegar residue supported nanoscale zero-valent iron. CEC, cation exchange capacity. TOC, total organic carbon. Eh, redox potential.

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treated by nZVI, which led to increase in soil iron (Sneath et al., 2013). The available iron in soil treated by nZVI@VR was obviously lower than that in the treatment of nZVI, which indicated that nZVI@VR can reduce the excessive release of iron during the remediation process. Similar results were observed by Su et al. (2016b) who used biochar-supported nZVI to remediate Cr(VI)contaminated soil. Therefore, based on the current results, nZVI@VR with a nZVI loading amount of 5% was used to prepare granular nZVI@VR in the following experiment.

3.6. Remediation effect of granular nZVI@VR on Cr-contaminated soil Considering the practical applications, nZVI@VR powder would be easily blown away when used as a soil amendment. When nZVI@VR particles become airborne they can have a negative impact on the health of exposed occupants. Moreover, powders can be difficult to pack and can be difficult to add into soil. Therefore, we considered converting nZVI@VR powder into nZVI@VR granules. To determine the effect of granular nZVI@VR on Cr immobilization, the leachability of Cr and Cr species in the soil were measured. The effect of granular nZVI@VR on Cr immobilization is shown in Fig. 5A. The immobilization efficiency of granular nZVI@VR on Cr(VI) and Crtotal increased with an increase in the amendment dosage. The results should be referred to the increased surface areas and reactive sites. The highest immobilization efficiency of 100% for Cr(VI) and 91.83% for Crtotal was achieved for granular nZVI@VR with a dosage of 10%. When the dosage of granular nZVI@VR was 5%, the immobilization efficiency of Cr(VI) was 91.99%, and Crtotal was 84.38%. However, from Fig. 3A, the immobilization efficiency of nZVI@VR (no granulation) on Cr(VI) was 98.68%, and Crtotal was 92.09%, which was higher than that remediated by granular nZVI@VR. These results indicated that the granulation of nZVI@VR reduced the immobilization efficiency of nZVI@VR on Cr in soil. This phenomenon could be because some nZVI was oxidized in the granulation process. The reaction time may could also be an important factor. Granular nZVI@VR after addition into soil needs some time to disperse to the soil, and then full contact with Cr. Therefore, prolonging the incubation time may increase the immobilization efficiency of Cr. A SEP was conducted to observe the change of Cr species in soil treated with granular nZVI@VR with different dosages. As can be seen from Fig. 5B, the Cr species in the control test were EX (42.97%), CB (10.50%), OX (22.12%), OM (11.52%), and RS (12.89%). After treated by granular nZVI@VR, the EX and CB in soil decreased, while OX, OM, and RS increased with increasing the granular nZVI@VR dosage. Especially in case where granular nZVI@VR dosage was 10%, the EX and CB decreased to 11.41 and 7.33%, while OX, OM, and RS increased to 37.68, 25.48, and 18.10%. Thus, granular nZVI@VR promoted the conversion of more accessible Cr (EX and CB) into less accessible forms (OX and OM). 4. Conclusions This study tested the feasibility of the composite material consisting of nZVI supported on VR (nZVI@VR) for the remediation of Cr(VI) in soil. Sedimentation tests denoted that the loading of nZVI on VR prevented them from aggregating and maintaining particle reactivity. nZVI@VR revealed higher immobilization capacity of Cr than plain VR and nZVI. Meanwhile, the addition of nZVI@VR had a positive effect on soil properties, and it can be used as an effective way of improving contaminated soil properties. Moreover, granular nZVI@VR also exhibited high immobilization efficiency of Cr. Therefore, ZVI@VR proved to be feasible in the remediation process of Cr-contaminated soil. Acknowledgements

Fig. 5. Remediation effect of granular nZVI@VR on Cr: (A) Immobilization efficiency; (B) Changes in Cr speciation. The different lowercase letters indicate significant difference between different treatments (p < 0.05). nZVI@VR, vinegar residue supported nanoscale zero-valent iron. EX, exchangeable; CB, bound to carbonates; OX, FeeMn oxide bound; OM, organic matter bound; RS, residual.

This work was supported by the National Key R&D Program of China (2017YFD0801300); the Key R&D Program of Shanxi Province of China (201703D211014); the Science and Technology Key Program of Shanxi Province of China (201603D21110-1); and the Higher Education Institution Project of Shanxi Province: Ecological Remediation of Soil Pollution Disciplines Group (20181401).

Please cite this article as: Pei, G et al., Vinegar residue supported nanoscale zero-valent iron: Remediation of hexavalent chromium in soil, Environmental Pollution, https://doi.org/10.1016/j.envpol.2019.113407

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